1. Technical Field
The present invention relates generally to radio frequency (RF) signal communications systems and circuitry, and more particularly, to transmit-receive RF front end integrated circuits for laptop computer applications.
2. Related Art
Wireless communications systems find application in numerous contexts involving information transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. Generally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. For wireless computer data networks, such standards include Wireless LAN (IEEE 802.11x), which is understood to be a time domain duplex system where a bi-directional link is emulated on a time-divided communications channel. Wireless LAN allows several computer systems within a local area to connect to an access point, which provides a link to the global Internet network. Due to the proliferation of mobile computing devices such as laptop computers, Wireless LAN networks may be found in a variety of public locations, including airports, cafes, and the like.
As is fundamental to any wireless communication system, laptop computers have a network interface card with a transceiver, that is, the combined transmitter and receiver circuitry, to enable Wireless LAN connectivity. The transceiver, with its digital baseband subsystem, encodes the digital data to a baseband signal and modulates the baseband signal with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the digital data represented by the baseband signal. An antenna connected to the transceiver converts the electrical signal to electromagnetic waves, and vice versa.
For improving performance of Wireless LAN networks, particularly in relation to increased data throughput and extended link range without additional bandwidth or transmit power, several enhancements over conventional single operating frequency and single transmission/reception schemes have been contemplated. One is the multiple input-multiple output (MIMO) system architecture. Higher spectral efficiency and link reliability is achieved by splitting a high data rate signal into several lower data rate signals, and transmitting each such lower data rate signal via separate antennas. The receiver end also has multiple antennas for receiving such separated signals, which are combined to a single data stream.
The latest IEEE Wireless LAN standard, 802.11n, employs an operating frequency on both the 2.4 GHz band, as well as the 5 GHz band. Furthermore, this standard also adds the aforementioned MIMO architectures. Thus, recent Wireless LAN enabled laptop computers are typically configured with a pair of antennas that are mounted on the inside of the cover. In most cases, the transceiver integrated circuit itself does not generate sufficient power or have sufficient sensitivity necessary for reliable communications. Thus, additional circuits referred to as a front end is utilized between the transceiver and the antenna. Generally, the front end circuit includes a power amplifier for boosting transmission power, and/or a low noise amplifier to increase receive sensitivity. Due to the differing operational parameters and tuning requirements for the 2.4 GHz band and the 5 GHz band, there are separate power amplifiers and low noise amplifiers for each.
The typical architecture for Wireless LAN network interface cards is comprised of a 5 GHz signal transmit line, a 2.4 GHz signal transmission line, a first 5 GHz signal receive line, a second 5 GHz signal receive line, a first 2.4 GHz signal receive line, and a second 2.4 GHz signal receive line, which are variously, selectively connected to a first antenna and a second antenna. In the front end circuit, the 5 GHz signal transmit line may be connected to a band pass filter and a power amplifier tuned therefor, and followed by a harmonics filter. Additionally, the 2 GHz signal transmit line may be similarly connected to a band pass filter and a power amplifier tuned therefor, and also followed by a harmonics filter. The outputs for the two different signal frequency harmonics filters are connected to a diplexer (L, H terminals), with the S terminal being connected to one terminal in a single pole, double throw switch. The other throw terminal thereof may be connected to an input of a dual band low noise amplifier, the output of which is connected to a diplexer. One of the terminals of the diplexer is connected to the 5 GHz signal receive line, and the other is connected to the 2.4 GHz signal receive line. The pole of the switch may be connected to a first one of the antennas, thus switching between the transmit and first receive lines of both the 5 GHz signal and the 2.4 GHz signal. The second antenna may be connected to a dual band low noise amplifier, the output of which is connected to another diplexer. One of the terminals of the diplexer is connected to the second 5 GHz signal receive line, while the other is connected to the second 2.4 GHz signal receive line.
The Wireless LAN network interface card is typically located on the main board of the laptop computer, and away from the antennas that are mounted on the clamshell cover. Thus, the foregoing implementations suffer from increased noise figures because of the long cable traces necessary. The losses attributable to the cable are typically around 2 dB. Additionally, the diplexers have an associated loss of around 0.5 dB to 0.7 dB, and the single pole, double throw switch likewise has an associated loss of around 0.5 dB to 0.7 dB. Accordingly, there is a need in the art for transmit-receive RF front end integrated circuits for laptop computer applications that reduce the receive chain noise figure, while minimizing current draw during signal transmission.